Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace wherein the feedstock, typically comprising crude or a fraction thereof optionally desalted, is heated sufficiently to cause thermal decomposition of the larger molecules. Among the valuable and desirable products include light olefins such as ethylene, propylene, and butylenes. The pyrolysis process, however, also produces molecules that tend to combine to form high molecular weight materials known as steam cracked tar or steam cracker tar, hereinafter referred to as “SCT”. These are among the least valuable products obtained from the effluent of a pyrolysis furnace. In general, feedstocks containing higher boiling materials (“heavy feeds”) tend to produce greater quantities of SCT.
SCT is among the least desirable of the products of pyrolysis since it finds few uses. SCT tends to be incompatible with other “virgin” (meaning it has not undergone any hydrocarbon conversion process such as FCC or steam cracking) products of the refinery pipestill upstream from the steam cracker. At least one reason for such incompatibility is the presence of asphaltenes. Asphaltenes are very high in molecular weight and precipitate out when blended in even insignificant amounts into other materials, such as fuel oil streams.
One way to avoid production of SCT is to limit conversion of the pyrolysis feed, but this also reduces the amount of valuable products such as light olefins. Another solution is to “flux” or dilute SCT with stocks that do not contain asphaltenes, but this also requires the use of products that find higher economic value in other uses.
In U.S. Pat. No. 4,446,002, the precipitation of sediment in unconverted residuum obtained from a virgin residuum conversion process is taught to be suppressed by blending the unconverted residuum with an effective amount of a virgin residuum having an asphaltene content of at least about 8 wt % of the virgin residuum at a temperature sufficient to maintain both residuum components at a viscosity of no greater than about 100 cSt (centistokes) during blending. Virgin residuum is the bottoms product of the atmospheric distillation of petroleum crude oil at temperatures of about 357 to 385° C.
In U.S. Pat. No. 5,443,715, steam cracked tar is upgraded by mixing with a “hydrogen donor”, preferably hydrotreated steam cracked tar, at or downstream of quenching of the effluent of a gas oil steam cracker furnace. In this regard, see also U.S. Pat. No. 5,215,649; and U.S. Pat. No. 3,707,459; and WO 9117230.
US 2005/0261537 discloses a process for cracking a heavy hydrocarbon feedstock containing non-volatile components and/or coke precursors, wherein a stripping agent is added to the feedstock to form a blend which is thereafter separated into a vapor phase and a liquid phase by flashing in a flash/separation vessel, and subsequently cracking the vapor phase.
Other references of interest include U.S. Pat. No. 3,622,502; U.S. Pat. No. 3,691,058; U.S. Pat. No. 4,207,168; U.S. Pat. No. 4,264,334; WO 91/13951; DE 4308507; and JP 58-149991.
The present inventor has surprisingly discovered that processing tar through a stripping tower produces an upgraded, deasphalted tar that is compatible with refinery fuel oil pools